The potassium-chloride cotransporter KCC2 decreases intracellular Cl- levels and renders GABA responses inhibitory. In addition, it has also been shown to modulate excitatory synapse development. In this project, we investigated how alterations of KCC2 expression levels affect these two key processes in cortical structures of a normal and/or epileptic developing brain.
First, we demonstrate that KCC2 expression is altered by early-life febrile status epilepticus. Febrile seizures affect about 5% of children during the first year of life. Atypical febrile seizures, particularly febrile status epilepticus, correlate with a higher risk of developing cognitive deficits and temporal lobe epilepsy as adults, suggesting that they may permanently change the developmental trajectory of neuronal circuits. In fact, the presence of a cerebral malformation predisposes to the development of atypical febrile seizures and temporal lobe epilepsy. The mechanisms underlying these effects are not clear. Here, we investigated the functional impact of this alteration on subsequent synapse formation and seizure susceptibility.
We analyzed KCC2 expression and spine density in the hippocampus of a well-established rodent model of atypical febrile seizures, combining a cortical freeze lesion at post-natal day 1 (P1) and hyperthermia-induced seizure at P10 (LHS rats). 86% of these LHS males develop epilepsy and learning and memory deficits in adulthood. At P20, we found a precocious increase in KCC2 protein levels, accompanied by a negative shift of the reversal potential of GABA (EGABA) by gramicidin-perforated patch. In parallel, we observed a reduction in dendritic spine size by DiI labelling and a reduction of miniature excitatory postsynaptic current (mEPSC) amplitude in CA1 pyramidal neurons, as well as impaired spatial memory. To investigate whether the premature expression of KCC2 played a role in these alterations in the LHS model, and on seizure susceptibility, we reduced KCC2 expression in CA1 pyramidal neurons by in utero electroporation of shRNA using a triple-probe electrode. This approach lead to reduced febrile seizure susceptibility, and rescued spine size shrinkage in LHS rats. Our results show that an increase of KCC2 levels induced by early-life insults affect seizure susceptibility and spine development and may be a contributing factor to the occurrence of hippocampal atrophy and associated cognitive deficits in LHS rats.
Second, we investigated whether KCC2 premature overexpression plays a role in spine alterations in the hippocampus. We overexpressed KCC2 in hippocampal organotypic cultures by biolistic transfection and in vivo by in utero electroporation. In contrast to what was previously published, we observed that both manipulations lead to a decrease in spine density in the hippocampus, as well as an increase in spine head size in vivo. In fact, it has been previously shown that overexpressing KCC2 leads to an increase of spine density in the cortex in vivo. To prove that this discrepancy is due to brain regional differences, we overexpressed KCC2 in the cortex by in utero electroporation, and similarly found an increase in spine density and length. Altogether, our results demonstrate for the first time, that alterations of KCC2 expression are brain circuit-specific. These findings highlights the obstacles we will face to find adequate pharmacological treatment to specifically modulate KCC2 in a region-specific and time-sensitive manner in epilepsy.